Coherent light generators – Particular beam control device – Modulation
Reexamination Certificate
1998-06-26
2003-02-25
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Modulation
C372S029011, C372S032000, C372S050121
Reexamination Certificate
active
06526075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a driving method usable to modulate a light source device for optical transmission according to an analog or digital signal and achieve the transmission, such as high-density wavelength division multiplexing (WDM) optical communications, while suppressing dynamic wavelength variation during fast modulations. The present invention also relates to a semiconductor light source device using such a driving method, and optical communication method and system using such a driving method or a device.
2. Description of the Related Art
As an oscillation polarization-mode selective or switchable dynamic single mode semiconductor laser, the following device has been developed and proposed (see, for example, Japanese Patent Laid-Open No. 2-159781 (1990)). The oscillation polarization mode of the polarization switchable laser is changed by a digital signal which is produced by superimposing a minute-amplitude digital signal on a bias injection current. The device is a distributed feedback (DFB) laser in which a distributed reflector, formed of a grating, is introduced into a semiconductor laser resonator cavity, and its characteristic of wavelength selectivity is utilized therein. In the device, a strain is introduced into an active layer of a quantum well structure, or the Bragg wavelength is located at a position shorter than a peak wavelength of its gain spectrum, so that gains for the transverse electric (TE) mode and the transverse magnetic (TM) mode are approximately equal to each other for light at wavelengths close to an oscillation wavelength, under a current injection condition near its oscillation threshold state. Further, a plurality of electrodes are arranged and currents can be unevenly injected through those electrodes. An equivalent refractive index in the cavity is nonuniformly distributed by such uneven current injection, and the laser oscillates in either the TE mode or the TM mode and at a wavelength which satisfies a phase matching condition and shows a minimum threshold gain. When the balance of the uneven current injection is slightly changed to vary a competitive relation of the phase condition between the TE mode and the TM mode, the polarization mode and the wavelength of the laser can be switched.
As a driving method of that device, a TE/TM switching method has been proposed in which the above minute-amplitude digital signal is superimposed on a bias current injected through one electrode of a two-electrode DFB semiconductor laser. This is the technique according to which bias points of currents injected through the two electrodes are appropriately set, the polarization-modulated output light from the laser is transmitted through a polarizer to be converted to intensity-modulated light, and TE-mode polarized light can be thus obtained with a large extinction ratio.
In the above-discussed digital polarization modulation system, however, when transmission of an analog signal, such as a video signal, is to be performed, the analog signal needs to be converted to a digital signal by an analog-to-digital converter and the signal is sent thereafter. Hence the number of components of the system increases in the analog video transmission. Further, the system could not cope with the expansion of a transmission capacity aimed by a multi-value digital transmission system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a driving method or apparatus for driving a polarization switchable laser to generate an intensity-modulated signal (typically, an intensity-modulated analog signal or a multi-value intensity-modulated digital signal), and a light source apparatus for performing optical transmission using such a driving method. Another object of the present invention is to provide an optical communication system or method for performing transmission of an intensity-modulated signal using such a light source apparatus, and a multiplexing optical communication system or method for performing transmission of multiplexed intensity-modulated signals using such a light source apparatus.
The present invention is directed to a driving method or apparatus for driving a semiconductor laser as a light source for optical transmission, wherein a semiconductor laser is used, which is provided with a structure in which oscillation light of the laser can be switched between two different polarization modes by modulating a current injected into a portion of a waveguide of the laser. The laser has characteristics that light in both the two modes is emitted within a polarization-mode switching range of the current and that intensities of the light in both the two different polarization modes respectively vary in accordance with an amount of the current within the polarization-mode switching range. The current injected into the portion of the waveguide is urged to a bias point current in the switching range, and a modulation signal current is superimposed on the bias point current such that the emitted light in at least one of the two modes is intensity-modulated in accordance with an amount of the modulation signal current. The above modulation signal current is an analog-signal modulation current, an analog-signal modulation current generated from a digital signal by a-modulation unit, a multi-value digital-signal modulation current generated from a digital signal by a modulation unit, or the like.
The principle of a driving method or apparatus of the present invention will be described referring to
FIG. 1
showing an example of a curve of I (a current injected into the portion of the waveguide) vs. L (light output) for TE-mode light and TM-mode light from the laser during DC driving. The curve shows outputs of the TE-mode light and TM-mode light appearing when a constant current is injected into an electrode on, a light emission side of the laser and a current injected into another electrode on a modulation side of the laser is changed. Here, the semiconductor laser has characteristics that light in both polarization modes is emitted within the switching range of the current and that intensities of the light in both polarization modes vary substantially linearly in accordance with an amount of the current within the switching range. Where the polarization switchable laser is a single mode laser, such as a DFB laser, both of the TE-mode light and TM-mode light oscillate in a single longitudinal mode at Bragg wavelengths (differing between the TE mode and the TM-mode) determined from the grating of the DFB laser or the like.
There exists a bias state in which the TE-mode light and TM-mode light coexist; near a switching point in the I-L curve (or in the switching range). The width of the current in the bias state (i.e., the width of the switching range) is indicated by &Dgr; I
sw
. When a modulation signal (here, an analog modulation signal is shown) is applied with an amplitude smaller than &Dgr; I
sw
, the TE-mode light is intensity-modulated with a large amplitude due to the characteristic that the I-L curve steeply rises in the switching range. Where a sign-inverted modulation signal is applied, desirably-modulated TM-mode light can be obtained.
In the above discussion, the example is selected which has the switching range wherein TM-mode light is switched over to TE-mode light as the bias current injected into the portion of the waveguide increases. However, when the I-L curve is selected which has the switching range wherein TE-mode light is switched over to TM-mode light as the bias current increases, intensity-modulated light in the TM-mode can be naturally obtained. In this case, the relation between TE-mode light and TM-mode light in
FIG. 1
is reversed by adjusting bias currents injected into respective waveguides.
More specifically, the following configurations can be adopted.
The semiconductor laser may be comprised of a distributed feedback semiconductor laser wherein a waveguide and a diffraction grating formed near the waveguide are arranged, the waveguide includes
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Ip Paul
Rodriguez Armando
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